In evaluating the characteristics of a liquid crystal and in various other applications, it has been required to investigate the response of a sample in the process of relaxation after stimuli are periodically given to the sample by an electrical means, laser, or other means. For this purpose, time-resolved spectrophotometry using a Fourier transform infrared spectrophotometer is available. Since this spectrophotometry is capable of measurement with high signal-to-noise ratio over a wide range of wave numbers, the method has been developed and utilized for years. Two types of time-resolved spectrophotometry exist. One uses a rapid scan interferometer, while the other employs a step scan interferometer.
FT IR spectrophotometry uses an interferometer consisting of a beam splitter, a moving mirror, and a fixed mirror. The moving mirror is moved to obtain an interferogram. For this purpose, the characteristics of the sample including the transmissivity must be constant during a period of obtaining an interferogram. If the characteristics vary, incorrect information will appear after the signal from the sample is Fourier-transformed. In time-resolved spectrophotometry, the period of the imparted stimuli must be longer than the period of the reaction process. Since periodic stimulus is given independent of the movement of the moving mirror, it is necessary to match the movement to the application of the stimulus. In the past, therefore, stimuli have been given in synchronism with a reference signal produced by the interferometer.
In time-resolved spectrophotometry using a rapid scan interferometer, stimuli are given in synchronism with the reference signal produced by the interferometer as described above, and interferograms are taken. Therefore, samples can be classified into the following three types according to the period of the reaction of each sample.
(1) Where the response to the stimulus is very slow (.tau.&gt;T).
That is, the period .tau. of the reaction or other similar change of state of the sample is longer than the period of one scan T of the moving mirror, i.e., the time taken for the interferometer to make one measurement to obtain one full interferogram.
(2) Where the response to the stimulus is relatively slow (t&lt;.tau.&lt;T).
That is, the period .tau. is longer than the sampling interval t at which the signal is sampled, for creating an interferogram.
(3) Where the response to the stimulus is very fast (.tau.&lt;t).
In the case (1) above, the period .tau. of response of the sample is longer than the period T of a scan of the moving mirror. Therefore, if the moving mirror is forced to make a faster scan as shown in FIG. 1(a), an interferogram is taken with some delay with respect to the imparted stimulus. A spectrum of the sample in a desired state can be obtained by Fourier-transforming the interferogram.
In the case (2) above, the period of response is shorter and so plural scans are made by the interferometer as shown in FIG. 1(b). In particular, stimuli are given at intervals t determined by the reference signal produced by the interferometer. The data which have the same delay time and are obtained from the first scan, the second scan, and so on are separately gathered and then organized into an interferogram. Thus, a spectrum of the sample in a certain state is derived.
In the case (3) above, the period .tau. of response is shorter than the sampling interval t. As shown in FIG. 1(c), a stimulus is repeatedly given in synchronism with the reference signal produced by the interferometer. Measurements are made with a given delay time .DELTA..tau.. An interferogram is created by organizing data having the same delay time. As a result, a spectrum of a sample in a certain state is obtained.
The prior art time-resolved spectrophotometry has several problems. Especially, in the case (3) above, synchronization with the reference signal produced by the interferometer is needed. Also, very quick measurements are necessitated. As an example, when measurements should be made at 100 points in a time interval of 100 .mu.s, each measurement must be made in 1 .mu.s. Therefore, a very large amount of data must be gathered and processed after each scan of the moving mirror. Furthermore, each set of data must be distinguished after the measurements. The data having the same delay time must be organized. Hence, the FT IR spectrophotometer must have a high-speed sampling mechanism, data organization function, and other functions which are not normally incorporated.
When the period of reaction is shorter than the sampling interval, the period between the end of the reaction and the next sampling is useless. This deteriorates the efficiency of measurement.
In the above-described example, stimuli are give in synchronism with the reference signal produced by the interferometer. It is difficult to realize such synchronization. Where naturally periodic stimuli are excited, no measurement can be made by the above-described method.